Modern vehicles contain electronic control units (ECUs) that are capable of performing on-board diagnostic (OBD) processes. Vehicles include connectors, such as OBD-II connectors, that allow external diagnostic tools to be coupled to the vehicle data buses to interact with ECUs. A diagnostic tool may query a vehicle for OBD test results as well as for real-time data. Moreover, a diagnostic tool may direct an ECU to perform some action. For example, a vehicle repair facility may connect to the OBD-II port to obtain information to determine why a check-engine light is illuminated. Some ECUs, such as the powertrain control module, perform critical functions. As such, those ECUs are secured to prevent accessing the diagnostic capability without authorization.